1. Field of the Invention
The present invention relates to apparatuses for performing secret communication in order to prevent illegal eavesdropping and interception by a third party. More particularly, the present invention relates to apparatuses for performing data communication through selecting and setting a specific encoding/decoding (modulating/demodulating) method between a legitimate transmitter and a legitimate receiver.
2. Description of the Background Art
Conventionally, in order to perform secret communication between specific parties, there has been adopted a structure for realizing secret communication by sharing original information (key information) for encoding/decoding between transmitting and receiving ends and by performing, based on the original information, an operation/inverse operation on information data (plaintext) to be transmitted, in a mathematical manner. FIG. 21 is a block diagram showing a structure of a conventional data communication apparatus based on the above-described structure. In FIG. 21, the conventional data communication apparatus has a structure in which a data transmitting apparatus 90001 and a data receiving apparatus 90002 a connected to each other via a transmission line 913. The data transmitting apparatus 90001 includes an encoding section 911 and a modulator section 912. The data receiving apparatus 90002 includes a demodulator section 914 and a decoding section 915.
The data transmitting apparatus 90001 and the data receiving apparatus 90002 previously share, with each other, first key information 91 and second key information 96 which have a common content. Here, information data 90 and the first key information 91 are inputted to the encoding section 911, and the second key information 96 is inputted to the decoding section 915, whereby the information data 98 is outputted from the decoding section 915. Further, for the sake of describing eavesdropping by a third party, FIG. 21 includes an eavesdropper's data receiving apparatus 90003 which containing an eavesdropper's demodulator section 916 and an eavesdropper's decoding section 917. Third key information 99, which is different from the first key information 91 and the second key information 96, is inputted to the eavesdropper's decoding section 917. Hereinafter, with reference to FIG. 21, an action of the conventional data communication apparatus will be described.
In the data transmitting apparatus 90001, the encoding section 911 encodes (encrypts) the information data 90 in accordance with the first key information 91. The modulator section 912 converts the information data encrypted by the encoding section 911 into a modulated signal 94 in a predetermined modulation method and transmits the modulated signal 94 to the transmission line 913. In the data receiving apparatus 90002, the demodulator section 914 demodulates, in a predetermined demodulation method, the modulated signal 94 which is transmitted via the transmission line 913. The decoding section 915, based on the second key information 96, decodes (decrypts) a signal demodulated by the demodulator section 914, thereby outputting information data 98.
Next, an action of the eavesdropper's data receiving apparatus 90003, in the case of the eavesdropping of the modulated signal 94 transmitted between the data transmitting apparatus 90001 and the data receiving apparatus 90002, will be described. In the eavesdropper's data receiving apparatus 90003, the eavesdropper's demodulator section 916 demodulates, in the predetermined demodulation method, the modulated signal 94 which is transmitted via the transmission line 913. The eavesdropper's decoding section 917 attempts, based on the third key information 99, decoding of a signal demodulated by the eavesdropper's demodulator section 916. Here, since the eavesdropper's decoding section 917 attempts, based on the third key information 99 which is different from the first key information 91, the decoding of the signal demodulated by the eavesdropper's demodulator section 916, the original information data 90 cannot be reproduced accurately. That is, since the eavesdropper's decoding section 917 does not share correct key information with the data transmitting apparatus 90001, the original information data 90 cannot be reproduced appropriately.
A mathematical encryption (or also referred to as a computational encryption or a software encryption) technique based on such mathematical operation maybe applicable to an access system described in Japanese Laid-Open Patent Publication No. 9-205420 (hereinafter referred to as patent document 1), for example. Patent document 1 discloses an access system having a PON (Passive Optical Network) constitution in which an optical signal transmitted from an optical transmitter is divided by an optical coupler, and distributed to optical receivers at a plurality of optical subscribers' houses. In the above-described access system, such optical signals that are not desired and aimed at another subscribers are inputted to each of the optical receivers. Therefore, each of the optical receivers encrypts the information data aimed at each of the subscribers by using key information which is different by the subscribers, thereby preventing a leakage/eavesdropping of the information data between the subscribers and realizing safe data communication.
The mathematical encryption technique is described in “Cryptography and Network Security: Principles and Practice” translated by Keiichiro Ishibashi et al., Pearson Education, 2001 (hereinafter referred to as non-patent document 1) and “Applied Cryptography” translated by Mayumi Adachi et al., Softbank publishing, 2003(hereinafter referred to as non-patent document 2).
However, in the case of the conventional data communication apparatus based on the mathematical encryption technique, it is theoretically possible for the eavesdropper to decrypt, even if the eavesdropper does not share the key information, a cipher text (a modulated signal or encrypted information data) by means of an all possible attack executing operations which use all possible combinations of key information, or by means of a special analysis algorithm. Particularly, improvement in processing speed of a computer has been remarkable in recent years, and thus there has been a problem in that if a new computer based on a novel principle such as a quantum computer is realized in the future, it is possible to eavesdrop on the cipher text easily within finite lengths of time.